Abstract

We describe the development and some preliminary results of fish-based, multimetric indices for assessing the ecological health of nearshore (<1.5 m deep) and offshore (>1.5 m) waters of the Swan Estuary, Western Australia. A suite of fish community characteristics (metrics) were first selected via a weight of evidence approach, based on their sensitivity to detect inter-annual change in estuarine condition. Seasonally-adjusted reference conditions for each selected metric were then established for each region of the Swan Estuary using 30 years of historical fish assemblage data, providing a best available standard against which the current and future health of the estuary may be assessed. The nearshore data first required standardisation to minimise the effects of gear-induced biases that were attributable to differences in the characteristics of the seine nets used to sample the nearshore fish fauna of this system since the mid-1970s. A net selectivity study was thus conducted during two seasons in two main regions of the estuary, to compare the compositions of fish samples collected using each of the different seine nets employed historically, i.e. 21.5, 41.5 and 133 m-long seines. The resultant data were subjected to generalised linear modelling techniques to derive net equivalence factors for quantitatively standardising fish abundance estimates, thereby minimising the effects of sampling biases. Scoring thresholds determined statistically from the 5th and 95th percentiles of the standardised, composite nearshore and offshore fish assemblage data sets enabled each metric in each sample to be scored according to the extent of its deviation from an appropriate, ‘best-available’ reference condition. We focus on some interpretations of preliminary trends in the resulting index values, to highlight both the benefits of historical data sets and the challenges they present when constructing reference conditions.